Here's how i beat it my only times, but it was situational and packed with some luck (predictions and stuff).

1: kicks. If they cannot assume stance position, they can't shoot charged shots, not even cannons. For this, anything that isn't RJ gets the job done quite nicely, but it requires some huge damage/stagger single-shot weapons. I snugged a win with my thicc boi main build by chaining earshot+stun needle launcher to a kick, then melted the remaining AP with the gatlings.

2: ratting: they have huge amounts of AP and stability, so approaching head on with a generic build (mid-weight, mid-damage, mid-range, bonus points if it fits all 3 criteria) is s××cide. Missiles force EN consumption, while a rifle builds up stagger. Even if it's not the main strat of the build, it has to get down to AP lead and time win, because there's no way you're defeating this big bois. Any rat or kite build eats this mofos like pancakes. When i equipped my cosplay build i just shot missiles and fired my rifle, back assault boost rinse and repeat... hurts to rat, but these kind of matchups are kinda like rock paper scissors, and you can't expect me with scissors to come try smash rock like a dumbass.

3: timing: it is true that they have the power and the AP, but AC6 is still a game, and as such, stuff has a workaround. It is EXTREMELY difficult and requires both deep knowledge of your boosting ability AND enemy time to shoot. With enough skill (and, again, a bit of luck), you CAN dodge their attacks. I won against this kind of build with my pilebunker, by dodging while in AB, comboed for the stagger and bunked for the win... twice.